Choosing o the right current limiting resistor of optocoupler

Thread Starter

ilginsarican

Joined Jul 13, 2017
91
Hello,
I use TLP293-4 optocoupler.
Here is the schematic:
I've tested this circuit at -40C .
Although the switch was not on, MCU read high signal(3V3).
How can this be possible?
I think R36(18K) too much.Should I choose the current limiting resistor according to this information(IF:10mA) in the picture below?
upload_2019-7-12_10-39-33.png
Schematic:
upload_2019-7-12_10-32-23.png
 

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ericgibbs

Joined Jan 29, 2010
8,722
hi ilgin,
The component values are incorrect.
eg: 24v/18k is only approx 1mA, not including the losses in the LED and the Opto emitter.

E
Corrected.

Use a 2k2 in place of the 18k that will give approx 10mA opto emitter current, which the d/s states
 
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LesJones

Joined Jan 8, 2017
2,317
I do not think your question is very clear. As you only seem to be describing the condition were the switch is open the value of R36 makes no difference. As no current is flowing through the LED the photo transistor will not be conducting so the voltage on it's collector will be 3.3 volts which if I understand you correctly is what you are seeing. Without finding the the data sheet for the CDBA340L-HF to find the transfer ratio I can't say if 18K is a good choice for the value of R36.

Les.
 

Thread Starter

ilginsarican

Joined Jul 13, 2017
91
hi ilgin,
The component values are incorrect.
eg: 24v/18k is only approx 1mA, not including the losses in the LED and the Opto emitter.

E
Corrected.

Use a 2k2 in place of the 18k that will give approx 10mA opto emitter current, which the d/s states
hi ilgin,
The component values are incorrect.
eg: 24v/18k is only approx 1mA, not including the losses in the LED and the Opto emitter.

E
Corrected.

Use a 2k2 in place of the 18k that will give approx 10mA opto emitter current, which the d/s states
Forward voltage of CDBA340 is 0.2V
From datasheet of TLP293 ,forward voltage of LED typically 1.25V and IF = 10 mA so (24-1,25-0,2)/0,01 equals of the resistor value.
Is this calculation correct?
 

Thread Starter

ilginsarican

Joined Jul 13, 2017
91
I do not think your question is very clear. As you only seem to be describing the condition were the switch is open the value of R36 makes no difference. As no current is flowing through the LED the photo transistor will not be conducting so the voltage on it's collector will be 3.3 volts which if I understand you correctly is what you are seeing. Without finding the the data sheet for the CDBA340L-HF to find the transfer ratio I can't say if 18K is a good choice for the value of R36.

Les.
Sorry, I've written wrong.
Although the switch was not on, MCU read low signal.I mean transistor is on.
Forward voltage of CDBA340 is 0.2V
Here is the current transfer ratio:

upload_2019-7-12_11-20-24.png
 

LesJones

Joined Jan 8, 2017
2,317
Re your corrected question. (Post #5) Either the coupler is faulty or the MCU is not reading the input correctly. Measure the voltage with respect to ground of pin 12 on the opto coupler. Disconnect the connection to the MCU input pin (Remove R38) and again check the voltage on pin 12. if the reading is still low then short pins 5 and 6 of the opto coupler. (This is in case there is some fault on the input side causing current through the input of the opto coupler.)

Les.
 

Thread Starter

ilginsarican

Joined Jul 13, 2017
91
Re your corrected question. (Post #5) Either the coupler is faulty or the MCU is not reading the input correctly. Measure the voltage with respect to ground of pin 12 on the opto coupler. Disconnect the connection to the MCU input pin (Remove R38) and again check the voltage on pin 12. if the reading is still low then short pins 5 and 6 of the opto coupler. (This is in case there is some fault on the input side causing current through the input of the opto coupler.)
At ambient temperature(25C) there is no problem,It is just happening at -40C,
 

ericgibbs

Joined Jan 29, 2010
8,722
hi ilgin
With a transfer ratio of <60% at 1mA emitter current and running at -40C, I would not expect it to work reliably.
E
 

Thread Starter

ilginsarican

Joined Jul 13, 2017
91
hi ilgin
With a transfer ratio of <60% at 1mA emitter current and running at -40C, I would not expect it to work reliably.
E
The first thought that came to my mind was that 18K was too high.
The datasheet of TLP293 confused my mind.
What is the correct CTR for calculation?
upload_2019-7-12_12-10-47.png
 

ericgibbs

Joined Jan 29, 2010
8,722
hi,
You are over thinking the problem.:)
For -40C operation I would use a 2k2 series resistor, as you can see the there is a wide variation in the CTR.
So to ensure reliable operation set the LED =10mA.
E
 

LesJones

Joined Jan 8, 2017
2,317
Correction to my post #3. I copied and pasted the wrong part number from the schematic. it should have been TLP293-4.
(I noticed this while looking for the data sheet for the opto coupler to see if there was a graph of current transfer ratio to temperature.)
It would have helped if you had told us in your first post that it worked at 25 Deg. C but not at -40 Deg. C.

Les.
 
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